Add and apply rubocop rules for empty lines

We were very inconsistent regarding these rules.

Personally I prefer no empty lines around blocks, clases, etc... as
recommended by the Ruby style guide [1], and they're the default values
in rubocop, so those are the settings I'm applying.

The exception is the `private` access modifier, since we were leaving
empty lines around it most of the time. That's the default rubocop rule
as well. Personally I don't have a strong preference about this one.


[1] https://rubystyle.guide/#empty-lines-around-bodies

app/helpers/verification_helper.rb

@@ -1,5 +1,4 @@
 module VerificationHelper
-
   def document_types
     [[t("verification.residence.new.document_type.spanish_id"), 1],
      [t("verification.residence.new.document_type.passport"), 2],
@@ -24,5 +23,4 @@ module VerificationHelper
 
     data_to_display + "*" * data_to_mask.size + "@" + email_provider
   end
-
 end